1. Field of the Invention
The present invention relates to a four-wheel steering motor vehicle, and more particularly to such a motor vehicle furnished with an antilock brake unit.
2. Description of the Relevant Art
In four-wheel steering motor vehicles, steering performance of the vehicle has been improved by steering both front and rear wheels according to angular movement of a steering wheel. To this end, it is known to detect a vehicle speed, to determine a steering angle ratio and a steering phase of the rear wheels to the front wheels according to the detected vehicle speed, then to calculate a target steering angle of the rear wheels according to a front wheel steering angle and the steering angle ratio and steering phase determined above, and finally to steer the rear wheels until the rear wheels assume the target steering angle.
Meanwhile, many modern vehicles are equipped with an antilock brake unit in an effort to improve stability of the vehicle when suddenly braked. The antilock brake unit detects rotational speeds of the respective wheels and reduces the pressure of a brake liquid to be supplied to a brake of the individual wheel when the rotating speed of the wheel is suddenly reduced due to the braking operation, thereby automatically regulating a slip coefficient of the wheel to a proper range so as to prevent the wheel from being locked. This prior art is exemplified by Japanese Patent Laid-Open Pubilication No. 253564/87, which discloses a four-wheel steering motor vehicle equipped with the antilock brake unit.
However, in the four wheel steering vehicle of the type in which the steering angle ratio of the rear wheels to the front wheels is variable depending on the vehicle speed, since the rear wheel steering angle varies due to the change of the vehicle speed even when the front wheel steering angle is kept constant, the rear wheel steering angle would change when the vehicle speed is reduced due to braking at the time of cornering. This steering angle change of the rear wheels would cause the vehicle to make a change of orientation so that smooth and comfortable maneuvering of the vehicle is difficult to achieve.